1. Field of the Invention
The present invention relates to a flyback converter system, and more particularly, to a flyback converter system capable of preventing two side switches from being turned on simultaneously.
2. Description of the Prior Art
Recently, because of rapid advances in electronic products, electronic products are shrinking in size and weight. Their functions are also increasingly diversified, but power consumption cannot rise simultaneously. Therefore, design of a power management integrated circuit (IC) becomes more complex, more efficient and demands a higher power density ratio per unit area. A high frequency switching power supply fits the above requirements well.
A flyback converter is one widely used switching power supply. Although the flyback converter may not require circuit isolation, the outputs and inputs of the flyback converter must be isolated in order to increase power efficiency, as well as for safety considerations. The flyback converter is a buck-boost converter with circuit isolation characteristics. A magnetic component of the flyback converter is used to transmit energy, and not as a transformer. In addition, the characteristics of the flyback converter are low cost, circuit design maturity, and simple structure. Therefore, the flyback converter is often used in design of an auxiliary power supply for supplying power to an entire system.
Please refer to FIG. 1. FIG. 1 is a diagram of a conventional flyback converter system 10. The flyback converter system 10 includes a power supply module 101 and a power receiving module 102. The power supply module 101 includes a voltage regulator 1011, a primary side 1012 of the flyback converter system 10, and a primary side switch 1013. The voltage regulator 1011 is used for stabilizing a voltage Vin. The primary side 1012 of the flyback converter system 10 is coupled to the voltage regulator 1011. The primary side switch 1013 is coupled between the voltage regulator 1011 and the primary side 1012 of the flyback converter system 10 for controlling a magnetic flux direction of the primary side 1012. The power receiving module 102 includes a secondary side 1021 of the flyback converter system 10, a secondary side switch 1022, a controller 1023, a load capacitor CL, a load resistor RL, a filtering capacitor Cdc, a resistor RMOT for adjusting turning-on time of the secondary side switch 1022, and a filtering resistor Rdc. The secondary side 1021 of the flyback converter system 10 is used for receiving power from the primary side 1012 of the flyback converter system 10; the secondary side switch 1022 is coupled to the secondary side 1021; and the controller 1023 determines whether the secondary side switch 1022 is turned on or off according to a drain voltage of the secondary side switch 1022.
When the primary side switch 1013 is turned on, electrical current passes through the primary side 1012 of the flyback converter system 10 gradually, and the flyback converter system 10 stores the power in the primary side 1012. Due to a variation of the magnetic flux of the primary side 1012, the primary side 1012 and the secondary side 1021 have opposite polarities. As shown in FIG. 1, both nodes A and B are at high voltages, so an inverse bias is imposed on a parasitic diode of the secondary side switch 1022, such that no power is transferred to the power receiving module 102. On the other hand, when the primary side switch 1013 is turned off, the magnetic flux of the primary side 1012 vanishes, resulting in a low voltage at the node B. Meanwhile, the parasitic diode of the secondary side switch 1022 is turned on, so there is power transferred to the power receiving module 102, current passes through the load resistor RL, and the load capacitor CL is charged.
Please refer to FIG. 2. FIG. 2 is a waveform diagram of the drain voltage of the secondary side switch 1022, a gate voltage of the secondary side switch 1022, a turning-on lasting time, and a turning-off lasting time when the power receiving module 102 has a light load (discontinuous current mode, DCM). When the magnetic flux of the primary side 1012 vanishes, resulting in voltage at B point being lower than a switch turning-on threshold VTH2, the secondary side switch 1022 is turned on and the gate voltage of the secondary side switch 1022 is converted from a low voltage to a high voltage. Because of the light load, current through the secondary side switch 1022 is low. When the magnetic flux direction changes again, the current through the secondary side switch 1022 reduces to zero. If the node B is at a voltage over a switch turning-off threshold VTH1, the secondary side switch 1022 is turned off, and the gate voltage of the secondary side switch 1022 is converted from a high voltage to a low voltage.
Please refer to FIG. 3. FIG. 3 is a waveform diagram of a drain voltage of the secondary side switch 1022, a gate voltage of the secondary side switch 1022, the turned-on lasting time, and the turned-off lasting time when the power receiving module 102 has a heavy load (continuous current mode, CCM). When the magnetic flux of the primary side 1012 vanishes, resulting in voltage at the node B being lower than the switch turning-on threshold VTH2, the secondary side switch 1022 is turned on, and the gate voltage of the secondary side switch 1022 is converted from a low voltage to a high voltage. Because of the heavy load, current through the secondary side switch 1022 is higher than the current in DCM mode. When magnetic flux direction changes again, the current through the secondary side switch 1022 is not reduced to zero. Because of a voltage drop between the drain and the source of the secondary side switch 1022, the switch turning-off threshold VTH1 must be set lower than the switch turning-off threshold VTH1 in DCM mode to avoid voltage of the node B being unable to rise over the switch turning-off threshold VTH1, the secondary side switch 1022 being unable to be turned off, the primary side switch 1013 and the secondary side switch 1022 being turned on simultaneously, and the flyback converter system 10 being damaged.
To sum up, the conventional flyback converter system 10 has two disadvantages. First, when the flyback converter system 10 is designed to operate in DCM mode, the flyback converter system 10 may switch from DCM mode to CCM mode due to sudden introduction of a heavy load. Meanwhile a voltage of the node B cannot rise over the switch turning-off threshold VTH1, so as to prevent the secondary side switch 1022 from being turned off, which would cause the primary side switch 1013 and the secondary side switch 1022 to be turned on simultaneously, and the flyback converter system 10 to be damaged. Second, setting the switch turning-off threshold VTH1 for the heaviest load condition for safety reasons may prevent the flyback converter system 10 from being damaged, however the secondary side switch 1022 may also be turned off too fast in a light load condition, decreasing efficiency of the flyback converter system 10.